Zero Fucks (1978)

At the beginning of the film, three medical students are hunting for a body to use for research purposes. They hear of a tramp that has been found dead. The body is in the mortuary and as no-one seems to know who he is; they decide to take the body for dissection without formal permission. When the body is placed on the operating table, one of the students recognizes the man, and scenes from his acquaintance with the dead man flash back. At the same time certain things begin to happen that they had not allowed for.